I'm trying to control a pneumatic solenoid valve with a mosfet, with the gate being controlled by a digital pin on the Arduino. When I first tried it, it seemed like the mosfet was always on, even w/out the +5v from the Arduino. I saw in an old thread that perhaps I need a 1Mohm resistor on the gate.
Anyway, I'm using an IRF734 mosfet, which has a Vgs of +/- 20, but a gate threshold of 2v min, 4v max. I thought a threshold voltage was a minimum voltage to turn on the mosfet, so how can it be 2 values?
Question is: I've read that logic level mosfets have a lower Vgs, but are weaker in almost every other category. I need this mosfet to switch quickly/reliably. Is there a way to make the IRF734 work? Or is there an easier way to accomplish this? Thanks.
How do you wire the Mosfet with the Solenoid.
You must wire the + to one end of coil the second end of the coil wired to Drain of the Fet. The source to GND and the gate to the arduino or another logic level.
This is an N-Channel Fet so you switch the negative Potential.
Anyway, I'm using an IRF734 mosfet, which has a Vgs of +/- 20, but a gate threshold of 2v min, 4v max. I thought a threshold voltage was a minimum voltage to turn on the mosfet, so how can it be 2 values?
It would be helpful to see a wiring drawing of your setup. You don't say what the voltage value for the solenoid is or how it is being supplied (external power supply?). If using a external voltage source for the solenoid it's ground must be wired to the Arduino ground.
A logic level mosfet is the only thing that will work if directly wired to a Arduino output pin. Most standard mosfets require +10vdc+ to completly turn on the mosfet, the threshold voltage is just the gate voltage where the mosfet just STARTS to conduct and not saturated on.
A gate to source resistor is not required but can be useful for situations where the Arduino is powered off but the external solenoid power is still on, the resistor will ensure that the mosfet stays off in that situation.
Yeah, sorry. The solenoid is operating at 19v from an external power supply; an old laptop brick that I had. You're right, I don't have a common ground. I'm going to run a voltage regulator off the same power supply to supply the Arduino with 10v, so I don't guess I would need that resistor.
You mentioned that only a logic mosfet would work directly with a digital pin on the Arduino. What could I do indirectly? Or you think that's a waste of time. For one, I'm having a little bit of trouble finding a logic level mosfet. Thanks.
If you don't use a logic level mosfet, then you will require a npn switching transistor that the arduino can drive via it's base and the npn collector resistor would wire to the +19v supply voltage. Then as the Arduino switches the npn converts it to a 0-19vdc gate drive signal. You also require a base series resistor from the arduino pin to the npn. This setup will introduce a logic inversion such that an arduino high output will turn off the mosfet and a low will turn it on.
Me, I only search out and buy logic level mosfets, my favorite.
Here is a nice high power N channel logic level mosfet that a user found recently:
Don't drive the gate with 19V whatever you do, that's way too close the absolute max spec of +/-20V gate-source voltage - a small spike on the supply will toast the device. Most non-logic level MOSFETs are designed to be driven at 10 to 12 volts.
This is a very poor choice MOSFET for this application, it has a massive Ron of 1.2 Ohms. This is because its a high voltage part. You want something rated at around 50 to 75V and nice low Ron (you don't say what current the solenoid takes do you?) Anyway a 60A part with Ron of less that 0.01 ohm ought to be fine Remember the current rating of MOSFETs is almost never a useful parameter, go by Ron and the power rating. If you are running a MOSFET anywhere near its max current rating it will need heatsinking and possibly a fan...
The IRF734 may need a heatsink, needs a gate drive circuit - easier to get a MOSFET that's logic level and up to the job.
The threshold voltage is the voltage it starts to conduct a little - the variation is manufacturing spread between devices. If the threshold is or could be 4V it is NOT logic level - it takes 2 to 3 volts above the threshold to be properly on - logic level devices have thresholds around 1V and tighter control of the spread.
Oh the other thing, power MOSFETs have built-in freewheeling diode, external diode not usually needed.
I understand that the mosfet I have is not logic level, I was just wanting some clarification on the specs. I knew I needed a mosfet, just didn't know anything about differences between them. I'm real new to, well, electronics in general, and just want to make sure I correctly understand everything I read/learn before I move on to something else.
The solenoid only pulls like 700mA, so I guess I'll hunt down a logic mosfet.
Thanks all.
Unsubstantial for a circuit, no, extremely small compared to what I'm used to dealing with in oilfield applications, yes. I know 700mA wasn't any problem at all for the mosfets I was looking at earlier. However, it appears that from the feedback that I've gotten that I most definitely want to go with a logic level mosfet. So is a logic level mosfet capable of 700mA going to be tough to find, or what are you saying? Thanks.
Right, which is why I wasn't too concerned and; why I've become so recently intriqued with circuits in general. Doesn't really cost anything but time. I see a lot of potential in arduino's applicability on my drilling rigs. Anyway, like I said, I'm real new to electronics and I do my best to read all I can on the web to avoid wasting yall's time on here, but if you could recommend 1 or 2 entry level books on circuits, I would greatly appreciate it.
What I need are books that explain "why". Why there's a resistor here, why there's a cap there. I already have "Getting Started in Electronics" by Forrest Mimms.
I see a lot of potential in arduino's applicability on my drilling rigs.
Be careful stuff on a drilling rig has to be built and tested to quite exacting standards that are not even hinted at here. Things like electromagnetic emissions and susceptibility to electromagnetic radiation and voltage surges and such like.
While not a beginners book as such it will take you from the basics right through to expert, it is good.
I build and operate my own rigs. You would be very suprised to know that the standards that are required by law aren't as exacting or a abundant as you might think, not in the US anyway. Most issues are more a matter of operator responsibility. Safety procedures are really the only issues that fall under the umbrella of OSHA authority.
Like I said, I build rigs for a living, and everything I'm planning on doing is not a problem I think arduino would be great for engine monitoring, depth tracking, pressure monitoring, automatic drilling...the list is limited only by the imagination.
Electromagnetic emissions only become a problem downhole, and components would be required to run in a monel drill collar, but everything I'm trying to achieve is at the surface.
Resistance to wear, water, and vibration is the primary concern for electronics on a drilling rig; that I know from experience.
Thanks Mike for the book and cautionary words. Have a good one.
Remember the current rating of MOSFETs is almost never a useful parameter, go by Ron and the power rating. If you are running a MOSFET anywhere near its max current rating it will need heatsinking and possibly a fan...